The majority of human cell types are polarized, i.e. they exhibit asymmetry, which is essential to their function. This includes epithelial cells that form barriers between the outside world and the underlying basement membrane and connective tissue, and make up most major organs. Establishment and maintenance of epithelialcell polarity depends on the precise targeting of proteins to the apical and basolateral plasma membrane domains usingvesicular transport pathways. Understanding the mechanismsthat underlie polarized trafficking is of fundamental importanceto understand functionand dysfunction of polarized cells. [unreadable] [unreadable] Vesicle transport pathways depend on the SNARE machinery for the final membrane fusion step. Syntaxins are the most central SNARE component as they directly interact with almost all other components. We have found that syntaxins 3 and 4 are specifically localized at the apical and basolateral domain, respectively, where they govern the fusion of incoming transport vesicles. [unreadable] [unreadable] The central hypothesis of this proposal is that syntaxins define the sites of vesicle exocytosis and that the apical and basolateral separation of syntaxins 3 and 4 is necessary for epithelial cell polarity. To test this, the targeting signals of syntaxins 3/4 will be identified, and the pathways that they follow en route to the apical or basolateral plasma membrane will be defined. Syntaxins 3/4 will be re-localized by mutagenesis of their targeting signals and by generation of chimeric molecules, and the consequences on specific membrane trafficking pathways and on cell polarity will be measured. The fusion sites of post-Golgi transport carriers containing GFP-tagged apical and basolateral markers will be monitored and we will ask whether they specifically co-localize with pre-assembled syntaxin 3/4 clusters. [unreadable] [unreadable] Collectively, this project is designed to provide answers to the following fundamental questions. (1) How are syntaxins 3/4 targeted in polarized MDCK epithelial cells? (2) Is the mutually exclusive localization of syntaxins 3/4 required for polarized cargo transport in epithelial cells and hence for the formation/maintenance of cell polarity? (3) Is the fidelity of polarized trafficking encoded in the specificity of v/t-SNARE interactions? (4) Do stable syntaxin clusters exist on the plasma membrane, and are they the sites of carrier fusion? (5) Do syntaxin 3/4 clusters specifically fuse only 'their' cargo carriers? (6) Is the localization of a syntaxin alone enough to establish a fusion site and create a surface domain?